Abstract: The present invention teaches a technique and provides for apparatus eminently useful for the continuous, simultaneous monitoring of the insulation quality of one or more pieces of high voltage ac electrical equipment, while the equipment is in service and connected to its normal high voltage source. The technique or method of the instant invention involves comparing a voltage, developed across a capacitive shunt at the capacitance tap of a transformer or bushing so equipped with a reference voltage taken from a voltage transformer or potential device connected to the same high voltage source.